14 INTEGRATION WITH RESPECT TO A POSITIVE MEASURE 189 0 and a mass n at the point l/Ğ. Then the sequence (//,( A)) is bounded for every open set A which is |ftn|-quadrable for all n, but the sequence of norms \\fj,n\\ is unbounded. Again, if we take ^n to be the measure defined by a mass n at the point l/n and a mass n at the point l/(n -f- 1), then the sequence (ftM(A)) is bounded for every interval A c [0, 1 ] (and therefore also for every finite subset A of [0, 1 ]), but the sequence of norms ||/xn|! is not bounded. 2. For each integer n J> 1, let En be an at most denumerable closed subset of I = ]0, 1[, and let */ be the family of component intervals of I En . Suppose that the maxi- mum length dn of the intervals J e Jn tends to 0 as* n -* -f oo . Let A be Lebesgue measure on I and let A be a A-measurable subset of I. Suppose that there exists a number k e ]0, 1 [ such that, for all n and all J e Jn , we have A(A n J) < AA(J). Show that A(A) = 0. (By using (13.7.9), show that A(A) ^ e + &(A(A) + e) for all s > 0.) 3. For each integer n > 0, the Farey series of order n is the set Fn of all rational numbers which, when expressed in their lowest terms plq, are such that Q^p^q^n, and arranged in increasing order. The distance between two consecutive terms of Fn is therefore ^l/n. (a) Show that if two rational numbers r~p/g and r'=p'/q' are such that VP' Ptf = ħ 1> tnen f°r every pair of integers (p"9 #") there exist integers jc, y such that p" px + p'y and q" qx + q'y. The fraction p*l<f belongs to the closed interval with endpoints r, r' if and only if x, ^ are of the same sign. (b) Deduce from (a) that if r=*p/q and r' p'lq' are two rational numbers belonging to the interval [0, 1], such that q > 0, q' > 0 and qp' pq' == ħ 1, then r and rf are consecutive elements of the Farey series Fsup(9, fl/> . Moreover the smallest of the integers w such that the open interval with endpoints r, r' contains a point of Fm is the integer q -h <?', and this open interval contains only one point of Fq+q,, namely (p +/>')/(<? + #') (c) Conversely, if r and r' are consecutive terms of Fn, show that qp' pqf = ħ1 (induction on /?). 4. For each x e I = ]0, 1], put /o(;t) = (1/x) [l/#], where [r] denotes the integral part of the real number /. lfp"(x) = p(p"""1(x)) is defined and nonzero, put qn + i(x) = [l//3n(x)] (a) For every Ğ such that p"(:c) is defined, show that An- !(*>*(*) -fAn(*) where An(jc) and Bn(jc) are integers >/i 1. The fractions An_i(Ar)/Bn_i(A:) and An(^:)/Bn(x) are two consecutive terms of a Farey series (Problem 3) and x belongs to the closed interval with these as endpoints. Deduce that pn(x) is defined for all/? ;> 1 if and only if x is irrational. The (finite or infinite) sequence of numbers qn(x) is called the continued fraction expansion of x, and the fractions An(x)/Bn(x) are the convergents to x. The An(x) and Bn(x) are constant on the complement of the denumerable closed set En of points x such that pn+l(x) is not defined, and on each of the component intervals of I En the function pn is monotone and varies from 0 to 1 . (b) Let A be Lebesgue measure on I and let A be a A-measurable subset of I consisting entirely of irrational numbers. Suppose that <pA is X-invariant with respect to p (SectionG in E x R.